In regions of the country subject to earthquake and hurricane events, it is becoming increasingly common to provide metal straps and anchors for affixing wood frame structures to their concrete foundations. For many years manufacturers such as Simpson Strong-Tie Company, Inc. have designed holdowns for use with bolts. (see Simpson catalog, Jan. 1, 1996 pages 14 and 15 which illustrates holdowns such as HD2A covered by U.S. Pat. No. 4,665,672). These holdowns are very effective, but their load rating is affected by the material which must be drilled out of the wood studs in order to receive the stud bolts which range in diameter from 5/8" up to 1". Moreover, some undesirable looseness is inherent in the connection by contractors who may inadvertently overdrill the bolt holes, or simply by the fact that wood shrinkage occurs after installation of the bolts.
The use of nails instead of bolts in holdowns has greatly reduced the shrinkage and looseness problem of bolts and has led to the development of strap holdowns as illustrated on pages 20, 22, and 23 e.g. of the Simpson catalog supra, (see e.g. U.S. Pat. No. 5,150,553.) The problem with holdowns which use nails is the fact that they must be very long to accommodate the many nails that are required. See e.g. Simpson catalog page 23 in which the HPAHD22-2p requires 23-16d nails and may be 22" to 42" in length. Many contractors now use nailing guns to drive the nails, but for the person who does not have a nailing gun, the prospect of driving 23 nails for each strap holdown means the expenditure of a great deal of energy driving the nails.
With the increasing use of powered drills, the feasibility of using wood screws as fasteners instead of nails and bolts is now a reality. The problem with screws, particularly for large loading in shear is that standard screws have several weaknesses. First, it was found that the heavy duty power drivers snapped the heads off a high percentage of standard screws before the clutch disengaged the drive at the end of the driving cycle when the head abruptly reached the immovable sheet metal connector plate. Second, those screws which had adequate unthreaded shank portions to resist the large shear loads, split the wood upon installation or shortly thereafter because the diameters of the unthreaded portions were larger than the bore made by the threaded portion of the screw. Third, adequate self drilling features were difficult to find in large size wood screws. Finally, existing screw fasteners with unthreaded portions adjacent the head which had smaller diameters to prevent wood splitting, were too loose. Looseness in standard screw fasteners between the unthreaded shank and the side of the bore hole which are subject only to pull out, is not a problem. Looseness, between the unthreaded shank portion and the side of the bore hole is a major problem when the screw fastener is subject to shear loads; particularly when the shear loads are cycling loads as they are in earthquakes and hurricanes. In such situations, each reversal of the shear loading tends to widen the bore opening until major loosening occurs and now the loads are impact loads which endanger the structure due to wood splitting.